Hypothyroidism has been found to produce a striking disturbance in the dimensions of the nephrons. In association with the disorder is growth that results in diminished body size. The kidneys are even more markedly affected and appear to be "atrophic" but controlled studies indicate that growth arrest, and not regression, is responsible. In this process, glomerular growth continues in proportion to overall body growth so that a dimensional glomerulotubular imbalance results. Measurements of single nephron glomerular filtration rate (Hanssen) and glucose reabsorptive activity have shown that this structural defect is compensated by a uniform vasoconstrictive reduction in filtration proportional to tubular capacity. Exploration of the role of the renin-angiotensin system in this response has disclosed a marked reduction in plasma renin substrate presumably due to an associated hepatic atrophy. Studies of hepatic function and the role of renin-substrate in the regulation of renal hemodynamics during hypothyroidism in the rat is in progress. BIBLIOGRAPHIC REFERENCES: Bradley, S.E., Coelho, J.B., and Sealey, J.E. Glomerulotubular adjustments in hypothyroidism in the rat - possible hepatorenal interaction. Abstracts, American Society of Nephrology 8: 73, 1975. Edwards, K.D.G., Coelho, J.B., and Bradley, S.E. Effects of hypothyroidism, undernutrition and diet on kidney weight and function in rats. Abstracts, American Society of Nephrology 8: 78, 1975.